An air conditioning system conventionally includes an evaporator, a compressor, a condenser and an expansion valve or other throttling device. As the liquid-vapor refrigerant mixture flows through the evaporator, heat is absorbed from a fluid being cooled and the refrigerant boils; resulting in a low pressure vapor that is compressed and then condensed in the condenser. This liquid refrigerant flows from the condenser to the expansion valve where its pressure and temperature are reduced and the liquid-vapor mixture issuing from the valve again flows to the evaporator.
A typical evaporator of the plate-fin type consists of a number of core elements connected together in parallel, with each core element formed from a pair of dished plates providing an enclosed cavity with an inlet at one side and an outlet at the opposite side; the core elements being spaced apart to allow air flow therebetween and heat transfer fins on the exterior of the core element plates extend into the air flow path. The inlets and the outlets of the core elements are connected together to provide an elongated inlet header and an elongated outlet header, respectively, with the core elements connected in parallel. Assuming that the refrigerant supply from the expansion valve is introduced in a lower inlet header, the supply enters the evaporator as a mixture of liquid and vapor. As this mixture travels through the lower header, the vapor portion is readily deflected upward to enter the core elements, while the liquid portion tends to continue travelling straight through the header due to its greater density and most of this liquid enters the last core element in its path. Thus, the preceding core elements are substantially "starved" of liquid refrigerant, resulting in a poor overall performance of the evaporator.
The problem can be somewhat alleviated by designing wide refrigerant passages into all parts of the evaporator to reduce the refrigerant velocity sufficiently so that the evaporator can be operated with the core elements flooded with liquid refrigerant, but without the liquid being carried over by the exiting vapor refrigerant. With such a low velocity, however, the advantage of a high velocity to improve heat transfer must be given up. This would not be a very serious loss in the main part of the evaporator under current design practice, since the heat transfer in the nucleate boiling regime, without the benefit of high convection, is still adequately effective.
Since the refrigerant must be superheated to some degree for the proper functioning of the thermostatic expansion valve, the low velocity of the refrigerant results in a disproportionately large portion of the evaporator devoted to the superheating section owing to the poor refrigerant heat transfer in this section. If the design practice changes to add more heat transfer area on the air-side, as would be desirable, the need to improve the refrigerant-side heat transfer, even in the main evaporating section, would intensify. Moreover, with the flooded condition of the evaporator, the quantity of refrigerant in the air conditioning system would appear to be critical. If there is too little refrigerant, the evaporator performance would suffer and, with too much, the liquid is likely to slug over into the compressor.
An alternative solution to the refrigerant distribution problem would be to connect the core elements in series rather than in parallel; however, such a construction may result in a high pressure drop in the evaporator due to the numerous reversals of the refrigerant flow direction at the ends of the core elements. This drawback will become most serious when the evaporator is designed for a high refrigerant velocity to improve the heat transfer.
A more serious problem, however, is that more core elements cannot simply be added to produce an evaporator of greater capacity. The simple addition of core elements would result in a rapid increase in the pressure drop due to the increase in the refrigerant flow rate through a given core element and the increase in the refrigerant path length. To reduce the pressure drop, the cross sectional area of the core element cavity must be increased. This means that, for optimum performance, an evaporator of each given capacity must be made from plates specifically designed for that particular capacity requiring one specific stamping die. Thus, the cost of tooling and inventory would be high.
The present invention overcomes the above enumerated problems of distribution of the refrigerant liquid-vapor mixture in parallel connected elements of a plate-fin evaporator.